Here it is on our old catamaran over 4 years ago. We left several other covered bumpers with the boat when we sold it, but we took the big one with us!
This time, I found some strange, stretchy, thick blue-jeans like material at the thrift store - exactly what I was looking for! Amazing how that works sometimes. It even matches our Sapphire Blue sunbrella material which I think of as a dusky-blue.
Easy-peasy - just rough-cut the size.
And sewed a "tube". Ended up taking another seam to make it nice and snug.
Don't want it to be droopy!
Then just made a pocket and threaded a string to tie it tight on the ends.
I did three smaller ones and the one large one. (I didn't bring all the bumpers home with me). The large one required a seam on one end because I didn't have enough material otherwise. Fun project while we waited out Hurricane Dorian at home!